Diagnostic elements are important components of clinically relevant analytical methods. This primarily concerns the measurement of analytes such as metabolites or substrates which are determined directly or indirectly, for example, with the aid of a specific enzyme for the analyte. The analytes are converted with the aid of an enzyme-coenzyme complex and subsequently quantified. In this process, the analyte to be determined is contacted with a suitable enzyme, a coenzyme and optionally a mediator and the coenzyme is physico-chemically changed by the enzymatic reaction. For example, the coenzyme may be oxidized or reduced by the enzymatic reaction.
If a mediator is used, it typically transfers the electrons released from the reduced coenzyme during the conversion of the analyte onto an optical indicator or the conductive components of an electrode so that the process can be detected photometrically or electrochemically. A calibration yields a direct relationship between the measured value and the concentration of the analyte to be determined.
The sterilizability of such test elements is of major importance, particularly when they are used for diagnostic purposes. Diagnostic test elements which are known from the prior art and used to determine blood glucose for example have the disadvantage that they have a low stability with regard to ionizing radiation or other physical or chemical agents that are conventionally used for sterilization. Similarly, certain features of these test elements, such as the enzyme system, may exhibit significant damage after sterilization. For example, certain test elements that are currently commercially available such as, by way of non-limiting example, the Accu-Check Active®, Accu-check Aviva® or the biosensors described in European Patent Publication No. EP 1 430 831 A1, which contain a pyrroloquinoline quinone (PQQ)-dependent enzyme and a mediator for electron transfer, show a significant loss of enzyme activity after sterilization with ionizing radiation.
A known measure that is used to compensate for damage to the enzyme system of biochemical test elements caused by sterilization is to overdose the enzyme system. European Patent Publication No. EP 1 293 574 B1 discloses electrochemical sensors which comprise a PQQ-dependent glucose dehydrogenase in combination with a phenothiazine mediator where the PQQ-dependent glucose dehydrogenase is used at a concentration of 20 enzyme units. After sterilization using electron radiation at a dose of 25 kilogray (kGy), sensors are obtained which, depending on the formulation of the enzyme system, have a high absolute amount of functional enzyme due to the overdosing of the enzyme system despite losses due to sterilization.
However, overdosing of the enzyme system in biochemical test elements has various disadvantages. For example, in addition to overdosing of the enzyme system being uneconomical and resulting in significantly higher production costs when producing consumables on a large industrial scale, high concentrations of enzymes in particular can cause problems in relation to solubility and/or viscosity which make it difficult to apply the enzyme to a suitable carrier.
One non-limiting object of the present application is to provide a stable biochemical test element for determining analytes such as glucose and that at least partially eliminates the disadvantages of the prior art. More particularly but not exclusively, in one aspect a test element that has a high proportion of active enzyme or active coenzyme and ensures good performance after sterilization and without overdosing the enzyme system is desired.